Plug-in for Adobe Photoshop Elements 13 that extends its functionality to include tools otherwise only available in Photoshop CC. This free c’t edition includes around half of the functions that you can unlock using the $12 full version of the plug-in.

A semi-professional, open source image processing software suite that includes layer support and a wide range of correction tools and filters. A user guide is available here.

The portable version of the semi-professional, open source GIMP image processing software suite that includes layer support and a wide range of correction tools and filters. Runs directly from USB or other memory media and requires no additional installation. A user guide is available here and you can download the source code here.

A plug-in for GIMP that contains a large number of predefined image filters and effects. The source code is available for download here.

Please note: The contents of the archive have to be copied to the GIMP plug-ins folder, which can usually be found under C:\Programs\GIMP-2.x\lib\gimp\2.0\plug-ins\. The G'MIC entry will be listed in the GIMP Filter menu when you restart the program.

An open source vector graphics editor with capabilities similar to Illustrator, CorelDraw, or Xara X. Uses the W3C standard scalable Vector Graphics (SVG) file format. Inkscape supports many advanced SVG features (markers, clones, alpha blending, etc.) and makes it easy to edit nodes, perform complex path operations, trace bitmaps and much more. The source code is available for download here.

Portable version of the Inkscape open source vector graphics editor. The source code is available for download here.

An open source RAW developer for use with digiKam, F-Spot, Shotwell or GIMP. Provides 16-bit import functionality for GIMP and can be used to process RAW images as well as TIFF, JPEG, BMP and PNG files.

A RAW converter that supports virtually all known RAW formats and includes tools for making significant improvements to overall image quality. Includes advanced white balance, exposure compensation, camera profiling, saturation, brightness and contrast algorithms.

A freeware RAW converter/editor that supports all current DSLR camera models. The source code is available for download here.

A lightweight and powerful photo and graphics editor that includes a range of pro-grade tools

Portable version of the StylePix free image editing program.

A plug-in for GIMP for individual or batch conversion of various RAW format image files. You can download the source code here.